Determination of Assay of Nitrous Oxide

Apparatus

The apparatus shown in Fig comprises a gas burette of 100-ml capacity, connected through a two-way tap at its upper end to two capillaries, one of which (A) is used to introduce the gas into the apparatus, the other (B) being connected to a vertical capillary arm to form a four-way junction. The descending arm of the junction is connected to a condenser (C) of about 60-ml capacity, and the right arm of the junction is connected to a mercury manometer (M). Tap D on the upper vertical arm of the junction opens to the air.

The lower part of the gas burette is fitted with a one-way tap connected by a rubber tube to a mercury reservoir. The upper part of the burette is graduated from 0 to 5 ml, and the lower part from 99.5 to 100.5 ml, both in increments of 0.1 ml.

Method

Close the three taps and immerse the condenser in liquid nitrogen, keeping the level slightly above the upper part of the condenser. By manipulating the two-way tap and the mobile reservoir create a partial vacuum in the apparatus, choosing an arbitrary pressure, Po, between 6.7 and 8 kPa (50 to 60 torr), accurately measured. This pressure must remain constant for 10 minutes to demonstrate that the apparatus is gas-tight. Open the two-way tap to tube A and completely fill the burette and tube A with mercury. Close the two-way tap. Connect a rubber tube through a suitable pressure-relieving device to the exit valve of the cylinder of the gas under examination and pass a current of the gas through the rubber tube for 1 minute.

Whilst the gas is still flowing, connect the rubber tube to the end of tube A and immediately open the two-way tap to tube A. Allow the specified volume of the gas to enter the burette by lowering the mercury reservoir. Disconnect the rubber tube and expel the gas from the burette by slowly raising the mercury reservoir above the capillary tube. Allow the specified quantity of the gas under examination to enter the burette by lowering the mercury reservoir and ensure that the pressure of the gas is equal to atmospheric pressure. Close the two-way tap.

Raise the mercury reservoir slightly above tube A and lower the level of the liquid nitrogen to the middle of the condenser. Carefully open the tap of the burette to connect with the condenser and allow the mercury to rise in the burette until it reaches the tap. Close the tap. Raise the level of the liquid nitrogen so as to totally immerse the condenser. Read the pressure and wait until it remains steady for 2 minutes. Place the mercury reservoir in its bottom position and open the tap of the burette to make the connection with the condenser.

Move the mercury reservoir until the manometer reading is the same as the initial pressure Po. Close the tap of the burette and, by means of the mercury reservoir, bring the pressure of the gas in the burette to atmospheric pressure. The number of ml of gas represents the non-condensable volume in the specified volume used. After each series of 10 determinations allow atmospheric air to enter by opening the tap D, remove the liquid nitrogen from the condenser and allow the condenser to warm to room temperature.

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